Variable current welding transformer



w, y 4 e e w, mu .m M, W l e 1 j 2 n 0 M W 7 V H r m WW.\\..\\\.NNQ 5 A. C. MULDER VARIABLE CURRENT WELDING TRANSFORMER Filed Oct. 2, 1950 June 30, 1953 Patented June 30, 1953 UNITED STATES PATENT OFFICE VARIABLE CURRENT WELDING TRANSFORMER 11 Claims.

control apparatus formed in a unitary structure to reduce the bulk of the equipment normally employed in arc welding systems and the like.

A further object of the invention is to provide a three-phase variable current transformer having direct current control windings which are adapted to prevent generation of undesirable alternating currents therein.

Another object of the invention is to provide a weldin transformer current varying means wherein control cores are introduced between the transformer windings for saturation by voltage and current feedback from the rectified transformer output.

The variable current welding" transformer is formed on a standard three-legged core, each leg having a primary winding energized by a source of three-phase power and a corresponding secoridary winding wound exteriorly of the primary and connected through a rectifier to anarc-we1ding electrode and to the work. The output of the transformer is controlled by means of three cores which are linked with the respective trancformer secondary windings and with two control windings, the latter windings being enersized for control core saturation by voltage and current feed-back from the rectifier. In another embodiment of the invention, the two control windings are replaced by two sets of seriesconnected windings wound on the respective control cores and balanced for cancellation of harmi ful alternating voltages induced therein from the secondaries. 7

Other objects and advantages will be set forth more fully in the following description of emsediments of the invention illustrated in the accompanying drawings.

In the drawings:

Figure 1 is a perspective view of the variable current transformer, with parts broken away to betterillustrate the structure;

Fig. Zlis a horizontal section taken along line 22 of Fig. 1;

Fig. 3 is a schematic wiring diagram showing the arc Welding system in which the transformer is incorporated; and

Fig. 4 is a fragmentary view, corresponding to Fig. 2, of an embodiment of the invention where-" in the control coils of the first embodiment are replaced by two sets of series connected coils.

Referring to the drawings and particularly to Figs. 1-3, the variable current transformer coinprises a core I formed of laminated magnetiz able material and having three parallel connect ed legs 2 spaced to receive primary windings 3 and secondary windings 4. The primaries 3 are wound one to each of the legs 2 and are independently connected across the various phases of a suitable source 5' of three-phase alternating current. The respective sets of windings 3 and 4 are preferably balanced to cause the magnetic fields produced by each phase of current to be of equal strength.

As best shown in Fig. 2 each of the sec ondaries 4 is wound around a core leg 2 exteriorly of its associated primary winding 3, so that the alternating primary voltage will be transformed by in-ductioninto secondary voltage for welding purposes. The secondaries are shown as connected in delta, with their outputs leading to a bank of rectifiers represented at 6 in Fig. 3. Although the'rectifiers 6 may be of any variety suitable for converting the three-phase transformer output into direct'current', it is preferable that barrier type selenium rectifiers be employed. In carrying out the invention, a control core I islinked with each of the transformer secondary windings 4, so that the inner leg 8 of each control core is disposed between the outer surface or" a prlmary 3 and the inner surface of the correspending secondary 4. The cores 1 are constructed of laminated magnetiz'able metal and are illustrated as being" rectangular in shape with their inner legs 8 connected through yokes to ouggr legs 9 disposed in parallel relation there W1 The control cores 1 are saturated by flux gem erated in a current reed-back control coil to in ter-linked with all three of the outer control core legs 9. To saturate the cores 1, direct current is fed back from the output of rectifiers 6 through a series connected control circuit comprising, in ad dition to the rectifiers 6 and control coil Ill, a selector switch ll, an arc welding electrode I2, and the work l3 to be welded.

The selector switch 1 i has its pivotally mounted arm 14 connected to an output terminal of the rectifiers and its various terminals l5 connected, respectively, to taps on the control coil i0. With this arrangement, the number of current feedback control coil turns through which the heavy welding current flows may be varied, and the degree of control core saturation varied accordingly.

Although only three selector switch terminals i5 and control coil taps are shown, the number of terminals and taps may be increased for finer adjustment of control core saturation. It is not necessary to have a large number of terminals and taps, however, since smooth variation of con trol core saturation may be obtained by the voltage feed-back arrangement next to be described,

The voltage feed-back control comprises a control coil Iii linked with the three control cores l and connected in series with a variable resistor ll across the output of the rectifiers 8. As illus= trated in Figs. 1 and 2, the voltage feed-back control coil it may be disposed adjacent the current feed-back coil around the outer legs e of the control cores.

In order to prevent the drawing of current from the rectifier, the coil i6 is formed of a large number of turns of relativelyhne wire. Ihe control coil It, on the other hand, is formed of a small number of turns of larger diameter wire to accommodate the heavy welding currents flowing therethrough under the current feedback control arrangement previously described.

The operation of the variable current welding transformer and system is as follows: Assume that the electrode i2 is out of contact with the worlr and that the variable resistor i7 is set so that current flow through the shunt control coil [6 is neglible. A strong alternating magnetic field is then set up in the unsaturated control cores l by flux from the secondaries -i, which held induces back into the secondary windings a voltage opposite in direction to the voltage induced therein from the primaries. The control cores thus opcrate as impedances in the transformer secondary circuits, to reduce the current output thereof to a relatively low value.

As soon as the electrode 12 contacts the work it, direct current will iiow through the control coil 0 to saturate or partially saturate the cores l,

with the degree of saturation desired and thus do termine the transformer output current depending upon the setting of selector switch 1 l. Saturation of cores 1 increases their reluctance and weakens the alternating magnetic field set up therein by flux from the transformer secondaries. Accordingly, the voltage induced back into the secondaries 4 by the control core field will be de creased and the transformer current output made correspondingly greater.

For fine variation of the degree of control core saturation, and thus the current output of the transformer, the variable resistor I? in the voltage feed-back circuit is manipulated to give the desired current value. In addition to its function of providing a means for fine adjustment, the voltage feed-back circuit through coil I6 is op" erable, by partially saturating the control cores even while the electrode I2 is out of contact with the work, to provide a large initial welding cur-- rent. The lag in welding current build up, which would be present if current feed-back alone were employed, may thus be compensated for and the striking of the welding are facilitated.

It is important, in order to prevent interier ence with the operation of the described Welding 4 current control, that no resultant alternating voltage be induced into the control coils l0 and it. Such voltages would drive currents alternate- 1y bucking and aiding the control currents and thus cause undesirable variations in the welding current output.

With the above described transformer, the control coils Ill and 16 remain free of alternating voltages since the degree phase displacement conventionally existing between the volt ages of three-phase power systems is such that the magnetic fields produced by the secondaries l completely neutralize each other and do not op erate to generate voltages in the control coils.

Referring to Fig. 4, a second embodiment of the invention .is illustrated wherein other means of controlling transformer output while preventing undesirable alternating voltage generation are utilized. These means comprise a set of three separate voltage feed-back control coils l8 wound on the respective control core legs 9 in place of the single coil It oi the first embodiment.

The coils [8 are connected in series with each other through leads l9 and are energized by a suitable source 20 of variable direct current, which current source may consist of the rectifier bank 6 and Variable resistor ll of the first embodiment.

In addition to the three coils l8 employed for voltage feed-back control, a set of control coils 2| may be utilized for current feed-back as in the case of the coil ii] of the first embodiment. Although they may be disposed at any point of the control cores 1, the coils 2| are shown as mounted, respectively, adjacent the coils IS on the outer legs 9 of the control cores.

Similarly to the coils I8, the three current feedback coils 2| are connected by means of leads 22 in series-circuit relation with each other. The coils 2| are energized through a feed-back source, represented at 23, which comprises a selector switch, not shown, connected in series with the electrode l2 and work l3 for welding current feed-back. By manipulating the selector switch, and by altering the taps to which leads 22 are connected or by the use of other leads, the amount of control core saturation by current feed-back through coils 2| may be controlled as in the first embodiment of the invention.

Except for the fact that two sets of three seriesconnected control coils are employed instead of the single coils l0 and [6, the transformer structure and the use of the transformer in a direct current welding system are the same as was described in connection with Figs. 1-3. The operation of the control coils in and 16 in varying the transformer current output is also the same as that of the previously described embodiment.

With respect to the prevention of alternating voltage generation in the control coils, however, a different principle is utilized. Instead of the magnetic field neutralization of the first embodiment, alternating voltages are induced into the respective coils ID and 16 but voltage neutralization is employed to cause the resultant alternating voltages in the two sets of series-connected coils to be zero.

To achieve voltage cancellation, the voltage feed-back windings l8 are accurately balanced with respect to location and number of turns so that equal voltages will be induced in each. Due to their 120 degree phase displacement, the induced voltages will efiectively cancel each th r to prevent alternating voltage interference with the welding current control function of the apparatus.

The three current feed-back windings 21 are also balanced, and harmful alternating voltages therein cancelled, as in the case of the voltage feed-back coils.

The apparatus described; in connection with both embodiments of the invention may be used for alternating as well as direct current welding, and constitutes simple and highly effective means of varying transformer; current output to an arc welding system. The use of current feed-back eliminates the necessity of providing a separate energy source capable of" producing the large amount of flux necessary for effective welding current control.

Various embodiments of the invention may be employed within the scope of the following claims.

I, claim:

1. A variable current transformer comprising a primary winding connected to a source of power, an output circuit including a secondary winding disposed in inductive relation with said primary winding, a control core formed of' magnetizable material entirely independent of said primary winding and linked in direct inductive relationship with said secondary winding, and electrical means to vary the reluctance of said control core and consequently the impedance, in the circuit of said secondary winding to control the current output of said transformer.

2. A variable current transformer comprising a transformer core of magnetizable material, a primary winding arranged on said core, and connected to a source of power, an output circuit including a secondary winding arranged on said core in inductive relation, with said primary, a magnetizable control core magnetically separate from said transformer core and linked in direct inductive relationship with said secondary winding external to said primary winding, and means to vary the reluctance of, said control core and consequently the impedance in the circuit of the winding associated therewith to control the current output of said transformer, said means comprising a pair of coils mounted on said control core and operable to variably saturate the same when energized by a source of variable direct current.

3. A variable current welding transformer comprising a transformer core of magnetizable material, primary and secondary transformer winding wound on said core in mutual inductive relation, a magnetizable control core separate from the transformer core and linked in direct inductive relationship with said secondary Winding to form an impedance in the circuit thereof, said control core being independent of said primary winding and having no effect upon the amount of exciting current flowing therein, and a control coil wound on the control core for connection with a source of variable direct current, said control coil operating to variably saturate said control core to alter the impedance in the circuit of said secondary winding and consequently the current output of said transformer.

4. A variable current welding transformer comprising a transformer core formed of magnetizable material, a primary winding wound on said core, a secondary windin wound on said core, a magnetiziable control core linked with said secondary winding and independent of said primary winding, one portion of said control core being inserted between the outer surface all) of said primary and the inner surface of said secondary, and a pair of control coils wound on s'aid control core outwardly of said secondary, said control coils operating tovariably saturate said control core to control the impedance in the circuit of said secondary and thusthe current output ofsaid transformer.

5; In an arc welding system, a transformer core formed of magnetizable material, a primary winding arranged on the core and connected to a, source of alternating current, a secondary winding arranged on the core in inductive relation with said primary and connected through a rectifier to an arc welding electrode and to the work to be welded, a magnetizable control core linked with said, secondary independentof said primary, a control coil wound on said control core and connected tosaid rectifier-for feedback therefrom, and means to control the amount of feed-back to said control coil and consequently the degree of saturation of said control core, whereby the impedance in the transformer secondary circuit is varied to control the current output to said welding electrode.

6. In an arc welding system, a transformer core formed of magnetizable material, a primary winding arranged on the core and connected to a source of alternating current, a secondary winding arranged on the core in inductive relation with said primary and connected to the input side of a rectifier, afmagnetizable control core linked with said secondary externally of said primary and forming animpedance in the circuit of said secondary, a current feedback control coil wound on saidv control core and connected in series-circuit relation with a welding electrode and the work to be welded across the output of said rectifier, a voltage feedback control coil wound on said control core and shunted across the output of said rectifier, and means to vary the amount of current andvoltage feed-back to said control coils, and thus the degree of control core saturation and the 01 .1- rent output of said secondary winding "I. In a three-phase transformer ofthe character described, a magnetizable transformer core formed with three parallel connected legs, each of said legs having a primary transformer winding wound thereon and a secondary transformer winding wound adjacent said primary, three magnetizable control cores linked one with each of said secondaries to form impedances in the circuits thereof, said control cores being external to said primary windings and having no effect upon the exciting currents flowing therein, a first control coil linked with said three control cores and operable when energized by direct current to saturate the control cores and decrease the impedances thereof, and a second control coil linked with said three control cores for fine adjustment of the degree of saturation thereof and thus of the current output of said transformer, said control coils being disposed with relation to said transformer windings to utilize magnetic field cancellation and prevent the induction of alternating voltages into said control coils.

8. In an arc welding system, a three-legged transformer core formed of magnetizable material, primary windings arranged one on each leg of said core and connected across the various phases of a source of three-phase alternating current, secondary windings arranged, respectively, in inductive relation with said primaries and connected through rectifying means to an arc welding electrode and to the work to be welded, three magnetizable control cores linked one with each of said secondaries externally of said primaries and forming impedances in the circuits of said secondaries, a control coil linking all ofsaid control cores and connected for feed-back from the output of said rectifying means, and means to control the amount of feedback to said control coil and consequently the degree of control core saturation and the current output of said secondaries, the relation between said control coil and the fields of said windings being such that no alternating current is induced into said control coil.

9. In a three-phase transformer of the character described, a magnetizable transformer core formed with three parallel connected legs, each of said legs having a primary Winding wound thereon and a secondary Winding wound adjacent said primary, three magnetizable control cores linked one with each of said secondaries externally of said primaries to form impedances in the circuits of said secondaries, and three series-connected control coils linked one with each of said control cores and operable when energized by direct current to saturate the control cores and decrease the impedances thereof to vary the output of said transformer, said series-ccnnected coils being balanced to cause cancellation of alternating voltages induced therein and thus prevent interference with the control of the transformer current output.

10. In an arc welding system, a three-legged transformer core formed of magnetizable material, primary windings arranged one on each leg of said core and connected across the various phases of a source of three-phase alternating current, secondary windings arranged, respectively, in inductive relation with said primaries and connected through rectifying means to an arc welding electrode and to the work to be welded, three inagnetizable control cores linked one with each ol said secondaries externally of said primaries and forming impedances in the circuits of said secondaries, three current feedback control coil linked, respectively, with said control cores and connected in series with each other and with said welding electrode and work across the output of said rectifier, and three voltage feed-back control coils mounted on said control cores and connected in series-circuit relation across the output of said rectifier, said control coils being balanced to cause cancellation of alternating voltages induced therein and thus prevent interference with the control of the welding current output of said secondaries.

11. In a three-phase transformer, magnetizable transformer core means, three primary, transformer windings arranged on said transformer core means and connected to a power source to be supplied by corresponding phases of a three-phase alternating current, three secondary transformer windings arranged on said transformer core means in inductive relation with the respective primary windings, magnetizable control core means linked in direct inductive relationship with said secondary windings independently of said primary windings, and means to vary the reluctance of said control core means and consequently the impedance in the circuits of said secondary windings to control the current output'of said transformer.

ALLAN C. MULDER References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,835,209 Dowling Dec. 8, 1931 1,928,848 Crout Oct. 3, 1933 1,997,234 Schmerber Apr. 9, 1935 2,668,393 Klinlzhamer Jan. 26, 1937 2,142,837 Edwards Jan. 3, 1939 2,265,930 Scott Dec. 9, 1941 2,454,582 Thompson Nov. 23, 1948 FOREIGN PATENTS Number Country Date 388,555 Germany Sept. 22, 1920 

